1. Field of the Invention
This invention regards an optical control device for broadband radar transmission and reception. It is applicable to the control of broadband electronic scanning antennas to ensure both the formation of a beam for transmission and the reception of a beam reflected by a target.
2. Discussion of the Background
An electric scanning antenna comprises many radiating elements that ensure both the transmission and the reception of an ultrahigh frequency signal. A transmission or reception beam is formed by all the signals transmitted or received by each element. To orient a beam in a given direction .theta., it is necessary to create time delays between signals transmitted or received by the various radiating elements. To obtain an analogous effect, it is known how to create a phase delay between these signals. The phase difference .phi.1-.phi.2 between the signals transmitted or received by two radiating elements is given by the following equation: ##EQU1##
where d represents the distance between the two radiating elements, f represents the frequency of the signals and c represents the speed of the light, the time delay created being ##EQU2##
The phase difference .phi.1-.phi.2 is equal to 2.pi.f(T.sub.1 -T.sub.2).
The preceding equation (1) highlights a major disadvantage residing in the fact that the phase difference depends on the frequency. Consequently, if the frequency varies, the sighting angle varies as well. This method for orienting a beam is therefore not suited to broadband radar. However ultrahigh frequency techniques do not allow us to create a time delay between the signals other than through the creation of the preceding phase difference, except to implement a device that is prohibitive from a size and cost standpoint. Effectively, a theoretically simpler solution would be to create a delay directly between the signals supplied to the different radiating elements, but that would require cumbersome and costly ultrahigh frequency circuits, due more particularly to the unavoidable dimensions imposed by the wavelengths in question.
The use of optical techniques allows us to overcome the aforementioned disadvantage by controlling the radiating elements directly through time delays, without requiring the artifice of phase differences, these delays being created in the optical domain. To that effect, optical control solutions for electronic scanning antennas have already been implemented. With regard to transmission, numerous optical control architectures have already been proposed in order to control the radiation pattern during transmission. An example of optical architecture is given in French patent No. 90 03386.
With regard to reception, beam formation using time delays requires a very significant dynamic of all the delays, still inaccessible to the optical components. A direct architecture based on the bi-directional operation of the control developed for transmission therefore does not seem possible in the short- or intermediate-term. To mitigate this disadvantage, a correlation architecture was defined in particular in accordance with the description of French patent No. 94 11498. However, this type of architecture is restricted to radars with a small bandwidth, typically 10 MHz.